1. Field
The present invention relates to a method of depositing a ruthenium (Ru) film, and more particularly, to a method of manufacturing semiconductor devices with Ru films having high density.
2. Description of the Related Art
Ruthenium films have been proposed to be used for thin film electrodes of semiconductor devices such as dynamic random access memories (DRAMs) and ferroelectric random access memories (FeRAMs). Ruthenium films to be used for the electrodes typically have a thickness of about 10 nm to about 20 nm.
A ruthenium film can also serve as a barrier against diffusion of copper in semiconductor devices. In such a case, a ruthenium film can be in contact with a copper layer. In addition, a ruthenium film may serve to increase adhesion of a copper layer or prevent electromigration of copper. In these cases, the ruthenium films may have a thickness of about 10 nm or less, and particularly, about 3 nm or less.
In certain cases, there is a need to prevent oxidation of a substrate during formation of a ruthenium film. In a case where a ruthenium film is used for copper interconnections having a double damascene structure, the ruthenium film is formed in a non-oxidation ambience in order to prevent oxidation of exposed copper on the bottoms of vias or a copper diffusion barrier film (a metallic film such as metal nitride).
Generally, a ruthenium film can be deposited using a chemical vapor deposition (CVD) method or a thermal atomic layer deposition (ALD) method. The CVD method includes simultaneously supplying a ruthenium source gas and oxygen gas (O2). The ALD method includes sequentially supplying a ruthenium source gas and oxygen gas (O2) (see T. Aaltonen, P. Alen, M. Ritala, and M. Leskela, “RUTHENIUM THIN FILMS GROWN BY ALD, Chemical Vapor Deposition, 2003, 9, No. 1, pp. 45-49). The ALD method may include repeating a gas supplying cycle. The gas supplying cycle can include sequences of pulses in each cycle, such as a ruthenium source gas supplying step, an inert gas reactor-purging step, an oxygen gas (O2) supplying step, and an inert gas reactor-purging step. Examples of the ruthenium source include Ru(Cp)2, Ru(EtCp)2, Ru(acac)3, Ru(thd)3, Ru(OD)3, and the like, wherein Cp is cyclopentadienyl, EtCp is ethylcyclopentadienyl, acac is acetylacetonate, thd is 2,2,6,6-tetramethyl-3,5-heptanedionate, and OD is 2,4-octanedionate. In addition, liquid ruthenium sources such as Ru(EtCp)2 and Ru(OD)3 are more suitable for mass production than solid ruthenium sources.
A ruthenium film formed by the CVD method using the ruthenium source gas and oxygen gas (O2) has a density of 6.6 g/cm3, which is much lower than a bulk density of ruthenium metal, that is, 12.37 g/cm3. The ruthenium film having a low density may be contracted or deformed in a subsequent thermal process. On the other hand, a ruthenium film formed by the ALD method has a density of 8.7 g/cm3, which is higher than the density of the ruthenium film formed by the CVD method.
A plasma-enhanced ALD (PEALD) method can also be used for forming a ruthenium film. The PEALD method includes repeating a gas cycle. The gas cycle can include a ruthenium source gas supplying step, an inert gas reactor-purging step, a plasma-activated ammonia gas (NH3) supplying step, and an inert gas reactor-purging step (see U.S. Patent Application Publication No. 2006-0177601, published Aug. 10, 2006, entitled “METHOD OF FORMING A RUTHENIUM THIN FILM USING A PEALD APPARATUS AND THE METHOD THEREOF,” by Park and Kang). A ruthenium film formed by the PEALD method has a density of 12.0 g/cm3, which is substantially equal to a bulk density of ruthenium metal, that is, 12.37 g/cm3.
Another PEALD method can be used for forming a ruthenium film. This method includes repeating a gas supplying cycle including a ruthenium source gas supplying step, an inert gas reactor-purging step, an oxygen gas (O2) supplying step, an inert gas reactor-purging step, a hydrogen gas (H2) supplying step, and an inert gas reactor-purging step (see Korean Patent Application No. 10-2001-0086955; Korean Patent Publication No. 10-2003-0056677; and U.S. Patent Application Publication No. 2005-0124154 A1 published Jun. 9, 2005). A ruthenium film formed by the PEALD method has a density of 12.0 g/cm3. In addition, the PEALD methods provide more uniform films than the CVD or thermal ALD method (hereinafter, collectively referred to as a non-plasma ALD method) which does not use plasma.
In order to be used as an electrode of a highly-integrated semiconductor device, a ruthenium film needs to have a relatively uniform thickness on a structure having a large aspect ratio. It is well-known that a film having good step coverage can be formed by the CVD method. However, a ruthenium film may not be formed uniformly in a narrow, deep structure by the CVD method.
In order to obtain good step coverage, a ruthenium film may be deposited by an ALD method which is performed by repeating a gas supplying cycle of sequentially supplying a metal source gas and a reaction gas.
However, the ruthenium film may not be formed on a metal oxide layer such as SiO2, Al2O3, HfO2, and ZrO2 immediately by the thermal ALD method. An initial period during which there is effectively no film deposition is called the incubation time of ALD. Typically, the incubation time of Ru thermal ALD on a metal oxide layer includes about 100 to about 200 ALD gas supplying cycles.
Because the incubation time is long and inconsistent, the thickness of the resulting ruthenium film may vary significantly after a fixed number of Ru thermal ALD gas supplying cycles. In addition, the long incubation time can decrease the productivity of the Ru ALD apparatus. On the other hand, in the PEALD method, since the incubation period is as short as several to twenty gas supplying cycles, the ruthenium film can be formed in a short period of time after the PEALD starts.
Now, examples of depositing a ruthenium film by a thermal ALD method or a PEALD method will be described with reference to FIG. 1. FIG. 1 is a graph showing relationships between the number of gas supplying cycles and thicknesses of ruthenium films formed by each of the thermal non-plasma ALD method and the PEALD method. Referring to FIG. 1, an initial film deposition speed of the PEALD method is higher than that of the thermal non-plasma ALD method.
In addition, in the PEALD method, the thickness of the deposited ruthenium film can be easily controlled by adjusting the number of the gas supplying cycles. Since the density of the deposited ruthenium film is 12.0 g/cm3, which is substantially equal to the bulk density of the ruthenium metal, that is, 12.1 g/cm3, the ruthenium film tends not to be deformed during subsequent thermal processing. In addition, the ruthenium film having a good adhesiveness and a smooth surface can be obtained. Therefore, the ruthenium film is suitable for an electrode having a very narrow structure which is required for a next generation memory device.
On the other hand, in the PEALD method which does not use oxygen gas, a ruthenium metal film having a high density can be formed without oxidation of a substrate. For example, in a case where a ruthenium film to be used as an electrode is electrically connected to titanium nitride (TiN) that is locally exposed to the substrate, the ruthenium film should be formed without oxidation of the substrate.
However, as shown in FIG. 1, at a deposition phase in which the film deposition is constant after the incubation period, the film deposition (about 0.04 nm or less per gas supplying cycle) in the PEALD method is slower than the film deposition (about 0.05 nm to 0.15 nm per gas supplying cycle) in the non-plasma ALD method. Therefore, it takes longer to form a thick ruthenium film by the PEALD method than by the non-plasma ALD method.
Another ruthenium film formation method is disclosed in Korean Patent Application No. 10-2003-0019258 and U.S. Patent Application Publication No. 2004/0224475, entitled “METHODS OF MANUFACTURING SEMICONDUCTOR DEVICES HAVING A RUTHENIUM LAYER VIA ALD AND ASSOCIATED APPARATUS AND DEVICES” by Lee Kwang-Hee et. al. The ruthenium film formation method includes a first step of forming a thin ruthenium seed film by an ALD method (which includes alternately supplying a source gas containing ruthenium and oxygen and another source gas containing hydrogen), and a second step of forming a ruthenium film having a desired thickness by a CVD method. The deposition speed per unit time of the CVD method is higher than that of the ALD method. As described above, the densities of the ruthenium films formed by the CVD method and the ALD method are 6.6 g/cm3 and 8.7 g/cm3, respectively. Therefore, the density of the ruthenium film formed by using the CVD method on the ruthenium film formed by the ALD method is less than 8.7 g/cm3.